Refractory sulphide ores are now a common source of precious metals. “Refractory” precious metal sulphide ores refer to ores and concentrates having low cyanide leaching efficiency (i.e., gold recovery). In refractory sulphide minerals, the precious metal-bearing sulphides are typically chalcopyrite, pyrite and arsenopyrite. To render refractory precious metal sulphide materials amenable to cyanide leaching, the sulphide matrix is destroyed.
Destruction of the sulphide matrix can be accomplished through a variety of oxidation methods, such as roasting, bacterial leaching, or pressure oxidation. In the pressure oxidation process, the precious metal-bearing sulphide minerals are oxidized in an autoclave at a high temperature (190-230° C.) and super atmospheric pressure while gaseous oxygen is injected into the pulp. Precious metals in the acidic pressure oxidation leach residues are commonly recovered by cyanidation or ammonium thiosulphate leaching. Prior to precious metal recovery, the autoclave discharge is either directly neutralized after cooling or subjected to a solid/liquid separation to remove acid and dissolved metals. If cyanidation is employed, the pH of the pulp must be increased to at least about pH 9.0 to avoid the formation of hydrogen cyanide.
Pressure oxidation reactions for sulphide minerals (pyrite FeS2 and arsenopyrite FeAsS) can be written ideally as:4FeS2+15O2+8H2O→2Fe2O3+8H2SO4 and2FeAsS+7O2+6H2O→2FeAsO4.2H2O+2H2SO4 Small amounts of iron and arsenic in the sulphide materials are also converted to the dissolved ferrous iron, ferric iron, arsenite and arsenate. Under these conditions, iron is precipitated in the autoclave as hematite (Fe2O3) and scorodite (FeAsO4.2H2O), and sulphuric acid is generated in solution. These two iron compounds are very desirable because they are chemically stable. It is possible to form other stable Fe—As compounds in the autoclave, depending on the temperature, the Fe/As ratio, and the acidity in the autoclave liquor. Because of their chemical stability, these compounds are inert during the subsequent neutralization and cyanidation steps and, therefore, do not consume expensive chemicals, such as lime.
Depending on the chemical conditions prevailing in the autoclave, other less desirable iron compounds can be formed. Examples of such compounds include basic iron sulphate, FeOHSO4, and jarosite, X Fe3(SO4)2(OH)6, where X is typically one of H3O+, Na+, K+, NH+4, ½Pb2+, and Ag+.
Jarosites and basic iron sulphates can be chemically instable. For example, in the autoclave discharge, basic iron sulphate can react with lime during pre-cyanidation neutralization to form ferric hydroxide and calcium sulphate:FeOHSO4+Ca(OH)2+2H2O=Fe(OH)3+CaSO4.2H2OAlso, some jarosites, particularly hydronium jarosite, react with lime during pre-cyanidation neutralization, to form ferric hydroxide and calcium sulphate:(H3O)Fe3(SO4)2(OH)6+2H2O+2Ca(OH)2→3Fe(OH)3+2CaSO4.2H2O
Although satisfactory gold recovery can be obtained by directly treating acidic pressure oxidation leach residues in an appropriate gold leaching and recovery process, silver recovery is frequently very poor. The most probable cause of poor silver recovery is the association of silver with refractory iron compounds (e.g., hematite, basic ferric sulphate, ferric arsenate and various forms of jarosite) formed by the hydrolysis and precipitation reactions that can occur during acidic pressure oxidation. The presence and relative quantities of these compounds can have a major impact on the method and economics of subsequent processes, and largely depends upon the nature of the starting material and the acidic pressure oxidation leach conditions. Generally, pressure oxidation under high acid conditions favours basic iron sulphate and possibly jarosite formation while low acid conditions favour hematite formation. When pressure oxidation is operated under conditions which favour hematite formation, the feed's sulphide sulphur content is converted to free sulphuric acid and dissolved metal sulphates in the solution phase (such as dissolved ferric sulphate), and, if calcium is present, as chemically stable and inert calcium sulphate in the solid phase. Neutralization of the free acid and dissolved sulphate salts in this type of autoclave discharge can be achieved inexpensively with limestone (CaCO3), which is a very cost-effective reagent. When the autoclave is operated under conditions that favour the formation of residues rich in basic iron sulphate and jarosite, it can have a significant negative economic impact on subsequent precious metal recovery operations, particularly the recovery of silver. Precipitates of basic iron sulphate and jarosite cannot be separated physically from the precious metal-containing solids. In addition, adequate neutralization of basic iron sulphate and/or jarosite can only be accomplished with stronger and more expensive neutralization agents, such as lime, CaO, or sodium hydroxide, NaOH.
U.S. Patent Application 2006/0133974, published Jun. 22, 2006, and entitled “Reduction of Lime Consumption When Treating Refractory Gold Ores or Concentrates” teaches the use of a hot curing process, as an effective method, prior to gold leaching, for reducing the cost of neutralizing acid residues from pressure oxidation. In this process, basic iron sulphate and free sulphuric acid, both contained in the autoclave discharge, react to form dissolved ferric sulphate according to the following equation:2FeOHSO4+H2SO4→Fe2(SO4)3+2H2OThis hot curing process has a residence time of 1 to 24 hours and a preferred temperature range of 85° C. to 95° C. Because the ferric sulphate-containing solution produced can be separated by solid/liquid separation techniques from the precious metal-containing residue, allowing time for basic iron sulphate to convert to dissolved ferric sulphate can reduce the consumption of expensive lime in the neutralization reaction of cyanidation feed in favor of inexpensive limestone. A further benefit of allowing time for the various components of the autoclave discharge to react with one another is that the strong ferric sulphate solution produced can be recovered and recycled to pre-treat the feed to the autoclave. Ferric ions in the recycled solution react with and oxidize sulphides in the autoclave feed material, thereby reducing the requirement and associated expense of oxygen in the autoclave process. In addition, any remaining acid in the recycle solution will react with carbonate minerals, when present in the autoclave feed material, and reduce the subsequent formation of carbon dioxide inside the autoclave and further improve the utilization of oxygen.
While the hot curing process is well suited to the treatment of pressure oxidation residues containing gold, it is less beneficial for the treatment of residues that also contain economically significant levels of silver. In practice, it has been found that the conditions used in the hot curing process favour the conversion of the silver contained in the residue to insoluble precipitates, possibly argentojarosite. The silver associated with this precipitate is extremely refractory to cyanide leach treatment resulting in silver extractions of less than 5 percent.
U.S. Pat. No. 4,632,701 describes an alkaline decomposition process that is an effective means of liberating silver from jarosites contained in pressure oxidation discharge residues, in which the alkali, usually slaked lime, reacts with the jarosites to form an alkali sulphate and an iron oxide, such as goethite. In the case of hydronium jarosite, the reaction with hydrated lime is:(H3O)Fe3(SO4)2(OH)6+2H2O+2Ca(OH)2→3Fe(OH)3+2CaSO4.2H2O
Other jarosites, including argentojarosite, also decompose in the presence of alkali. To drive the reaction to the right, the slurry pH of the pressure oxidation residue is increased to pH 10 or pH 10.5, and the slurry maintained at a temperature ranging from 80° C. to 95° C. for a time ranging from 0.5 to 4 hours. If the alkali carbonate step is employed, the total residence time increases to approximately 6 hours. The alkaline slurry is then subjected to a silver recovery treatment, such as cyanidation, without liquid-solids separation.
It has been found that liberating silver from pressure oxidation residues may require uneconomically high lime consumptions, with the cost of the lime far exceeding the value of the silver liberated. Lime requirements of 100 to 200 kg/t of ore are not unusual, and depending on the cost and amount of alkali reagents and the silver grade, the process may not be economically justifiable.
As a result, as of yet, there is no satisfactory process which offers an economic method of recovering silver by pressure oxidation from refractory sulphide ores.